In cobalt-catalyzed high-pressure hydroformylation, carbon monoxide and hydrogen are added onto the C═C double bond of the olefin in the presence of the catalyst metal cobalt which is active homogeneously as HCo(CO)4, forming an aldehyde. The aldehyde has one more carbon atom than the olefin. The reaction is generally carried out in a reactor at temperatures of from 120 to 240° C. under a synthesis gas pressure of from 150 to 400 bar.
Multistage processes have been developed for cobalt-catalyzed high-pressure hydroformylation and these allow the catalyst leaving the reactor with the hydroformylation products to be separated off from the hydroformylation products and be recirculated to the hydroformylation reaction. Such a process can comprise the 4 process steps of precarbonylation, catalyst extraction, olefin hydroformylation and cobalt recovery.
In the precarbonylation, the catalyst HCo(CO)4 required for the hydroformylation is firstly produced from an aqueous cobalt salt solution comprising, for example, cobalt formate or cobalt acetate by reaction with carbon monoxide and hydrogen. In catalyst extraction, the catalyst produced in the first process step is extracted from the aqueous phase by means of an organic phase, preferably the olefin to be hydroformylated. After phase separation, the organic phase loaded with the catalyst is fed to olefin hydroformylation. In cobalt recovery, the organic phase of the reactor discharge is freed of the cobalt carbonyl complexes in the presence of process water, which can comprise formic acid or acetic acid, by treatment with oxygen or air. Here, the catalyst is oxidatively destroyed and the cobalt salts obtained are back-extracted into the aqueous phase. The aqueous cobalt salt solution obtained from the cobalt recovery step is recirculated to the first process step, viz. precarbonylation. The precarbonylation, catalyst extraction and olefin hydroformylation can also be carried out in a single-stage process in the hydroformylation reactor.
It is known that HCo(CO)4 is stable only at a high partial pressure of the CO at the high temperature prevailing in the hydroformylation reactor (see New Synthesis with Carbon Monoxide, J. Falbe, Springer Verlag 1980, page 17, FIG. 1.9). Zones in which mixing is greatly below average are generally formed in the reactor. Particularly in less strongly mixed zones, the temperature can be elevated and/or the partial pressure of the CO can be lowered, so that HCo(CO)4 decomposes in these zones. The decomposition results in precipitation of metallic cobalt, forming cobalt deposits. The cobalt deposits can lead to mixing in the reactor being impaired. The poorer mixing in turn promotes further formation of cobalt deposits. Ultimately, the decreasing mixing in the reactor leads to a reduction in the yield of hydroformylation product.
The mechanical removal of cobalt deposits can be effected by, for example, use of a high-pressure water jet with the reactor lid open. However, the opening of the reactor lid and the subsequent pressure-tight reclosure is associated with a high engineering outlay.
It is known from EP 0 024 761 A1 that transition metal deposits which form on the interior walls of a hydroformylation reactor can be removed by cleaning with corrosive liquids, e.g. aqueous nitric acid.
Aqueous nitric acid can be introduced into the reactor without opening of the reactor lid. It can be introduced into the reactor through openings such as pipe sections. Dissolution of cobalt deposits by aqueous nitric acid forms an offgas comprising nitrogen oxides.